1. Technical Field of the Invention
The present invention relates to the generation of a voltage ramp at the terminals of a capacitor and, in particular, to the generation of a voltage ramp for use in a slope compensator unit of a switching mode power supply unit. Although not intended to be limiting of the invention, the present invention applies with advantage to battery-powered portable systems, especially remote terminals of wireless communication systems (for example, cellular mobile telephones).
2. Description of Related Art
A switching mode power supply unit generally comprises a core, which is formed of switches, an inductor and a capacitor, and a control loop, which controls the core to obtain the required output voltage.
In a “current mode” switching mode power supply unit, the control loop conventionally includes a “slope compensator unit,” which is necessary for maintaining the stability of the control loop for duty cycles exceeding 50%.
The slope compensator unit is in fact a current generator delivering a linear ramp. The unit is synchronized to a control signal in which the spacing between edges is related to the duty cycle between the required output voltage and the power supply voltage.
The slope compensator unit itself comprises two main components:                a constant current source charging a capacitor (referred to as the “ramp” capacitor) when the control signal is at 0, for example, which produces a voltage ramp at the terminals of the ramp capacitor; when the control signal is at 1, the ramp capacitor is discharged and there is no voltage at its terminals, and        a voltage/current converter which converts the voltage ramp at the terminals of the ramp capacitor into a current.        
To limit the surface area in an integrated circuit occupied by the ramp capacitor in the slope compensator unit, a component is advantageously used having a high capacitance per unit surface area.
If the power supply voltage is subject to transient variations, a temporary loss of regulation is observed at the output of the switching mode power supply unit, and is a direct consequence of the behavior of the slope compensator unit.
The reason for this is: as the ramp capacitor of the slope compensator unit is referenced to the power supply voltage, in the event of transient variations in the power supply voltage a transient current is produced in the capacitor and is added to or subtracted from the current supplied by the current source which is intended to charge the capacitor with a constant current.
The final current ramp delivered by the slope compensator unit is therefore increased or decreased in the event of fast transient variations of the power supply voltage, which causes said temporary loss of regulation at the output of the switching mode power supply unit.